Friday, 28 February 2014

This weekend's plot encompasses almost the entire field of direct detection of WIMP dark matter:

It shows the existing and projected limits on the scattering cross section of dark matter on nucleons. LUX -- a 370 kg xenon detector -- currently holds the leader position for dar matter masses above 6 GeV and promises to improve the limits by another factor of a few next year. The Xenon collaboration on the other side of the Atlantic is already preparing a nuclear response in the form of a 3 ton detector, to which LUX will retaliate with a 5 ton Led Zeppelin, or maybe LUX-Zeplin. Meanwhile, the SuperCDMS experiment will secure a monopoly in the low-mass region. But the arms race cannot go on forever, as direct detection experiments will inevitably hit the neutrino wall. That is to say, they will reach the sufficient sensitivity to observe nuclear recoils due to elastic scattering of solar and atmospheric neutrinos. That will constitute an irreducible background to dark matter searches (unless directional detection techniques are developed). And so it'll all come to a bitter end: sometime in the next decade WIMP detection experiments will be downgraded to neutrino observatories.

The plot borrowed from this talk who itself borrowed it from somewhere.

Wednesday, 26 February 2014

It is practically decided that the next high-energy collider we build is the ILC - an electron-positron linear collider in Japan starting at the center-of-mass energy starting of 250 GeV. But it's already high time to think of the following step. Two weeks ago a kickoff meeting on future circular colliders took place in Geneva, so it is now officially permitted to dream of a 100 TeV collider.

Particle theorists already started bidding for the most amazing discoveries that await us there. We'll surely find supersymmetry, dark matter, extra dimensions, hairy black holes, a fistful of Higgs bosons, a few bosons more, and the beard of John Ellis. This game seems to ignore the fact that circumstances have changed after the first run of the LHC. Whatever the energy of the new machine, 30, 100, 1000 TeV, we currently do not have a no-lose theorem for a discovery of new particles.

This is an unprecedented situation in collider physics.
For each new collider at the energy frontier in the last 30 years such a firm no-lose theorem existed.
In particular,

The LHC was certain to discover the Higgs boson,

The Tevatron was certain to discover the top quark,

LEP was certain to discover supersymmetry,*

The SppS was certain to find the W and Z bosons.**

* There is this apocryphal story of a prominent Italian physicist who vowed to cut off his testicle if superpartners are not found at LEP. It isn't clear whether he had balls to do it. ** Older readers may remember that the SppS was certain to also discover the top quark and supersymmetry. Just in case, Rubbia initially discovered both of these along with the W boson.

Now it's all different: we have a consistent and experimentally established theory of fundamental interactions that in principle may be valid all the way to the Planck scale. There is no strong case that new particles have to show up at accessible energies, and several precision measurements hint to the contrary. In this situation, banging the drum, like we did before the LHC, is dangerous since we may not have a big discovery to cover for that later. It seems wiser to focus on more certain returns from a 100 TeV collider. That is to say, even if there's no new particles within reach, what shall we learn? Of course, for all of us in the particle community, exploring a new unchartered territory is interesting per se. But we need more to convince others. The argument that we need a 100 TeV collider to prove that electroweak fine-tuning is larger than 0.01% is good. As a joke to relax the atmosphere. Certainly, the case for the new collider is stronger than that. Some ideas that are being bandied around are precision Higgs physics, double Higgs production, rare Higgs and top decays, non-perturbative electroweak physics, or WW scattering. These topics can be made more concrete and several more items can be added to the list.

Or we can study the impact of the 100 TeV collider on the CMSSM parameter and hope that no one will call the bluff...

Saturday, 22 February 2014

To force myself into a more regular blogging lifestyle, I thought it would be good to have a semi-regular column. So I'm kicking off with the Weekend Plot series (any resemblance to Tommaso's Plot of the Week is purely coincidental). You understand the idea: it's weekend, people relax, drink, enjoy... and for all the nerds there's at least a plot.

For a starter, a plot from the LHC Higgs Cross Section Working Group:
It shows the Higgs boson production cross section in proton-proton collisions as a function of center-of-mass energy. Notably, the plot extends as far as our imagination can stretch, that is up to a 100 TeV collider. At 100 TeV the cross section is 40 times larger compared to the 8 TeV LHC. So far we produced about 1 million Higgs bosons at the LHC and we'll probably make 20 times more in this decade. With a 100 TeV collider, 3 inverse attobarn of luminosity, and 4 detectors (dream on) we could produce 10 billion Higgs bosons and really squeeze the shit out of it. For the Higgs production in association with a top-antitop quark pair the increase is even more dramatic: between 8 at 100 TeV the rate increases by a factor of 300 and ttH is upgraded to the 3rd largest production mode. Double Higgs production increases by a similar factor and becomes fairly common. So these theoretically interesting production processes will be a piece of cake in the asymptotic future.

Monday, 17 February 2014

The title of this post is purposely over-optimistic in order to increase the traffic. A more accurate statement is that a recent analysis of X-ray spectrum of galactic clusters claims the presence of a monochromatic 3.5 keV photon line which can be interpreted as a signal of a 7 keV sterile neutrino dark matter candidate decaying into a photon and an ordinary neutrino. It's a long way before this claim may become a well-established signal. Nevertheless, imo it's not the least believable hint of dark matter coming from astrophysics in recent years.

First, let me explain why one would anyone dirty their hands to study X-ray spectra. In the most popular scenario the dark matter particle is a WIMP -- a particle in the GeV-TeV mass ballpark that has weak-strength interactions with the ordinary matter. This scenario may predict signals in gamma rays, high-energy anti-protons, electrons etc, and these are being searched high and low by several Earth-based and satellite experiments. But and in principle the mass of the dark matter particle could be anywhere between 10^-30 and 10^50 GeV, and there are many other models of dark matter on the market. One serious alternative to WIMPs is a keV-mass sterile neutrino. In general, neutrinos are dark matter: they are stable, electrically neutral, and are produced in the early universe. However we know that the 3 neutrinos from the Standard Model constitute only a small fraction of dark matter, as otherwise they would affect the large-scale structure of the universe in a way that is inconsistent with observations. The story is different if the 3 "active" neutrinos have partners from beyond the Standard Model that do not interact with W and Z bosons -- the so-called "sterile" neutrinos. In fact, the simplest UV-complete models that generate masses for the active neutrinos require introducing at least 2 sterile neutrinos, so there are good reasons to believe that these guys exist. A sterile neutrino is a good dark matter candidate if its mass is larger than ~keV (because of the constraints from the large-scale structure) and if its lifetime is longer than the age of the universe.

How can we see if this is the right model? Dark matter that has no interactions with the visible matter seems hopeless. Fortunately, sterile neutrino dark matter is expected to decay and produce a smoking-gun signal in the form of a monochromatic photon line. This is because, in order to be produced in the early universe, the sterile neutrino should mix slightly with the active ones. In that case, oscillations of the active neutrinos into sterile ones in the primordial plasma can populate the number density of sterile neutrinos, and by this mechanism it is possible to explain the observed relic density of dark matter. But the same mixing will make the sterile neutrino decay, as shown in the diagrams here. If the sterile neutrino is light enough and/or the mixing is small enough then its lifetime can be much longer than the age of the universe, and then it remains a viable dark matter candidate. The tree-level decay into 3 ordinary neutrinos is undetectable, but the 2-body loop decay into a photon and and a neutrino results in production of photons with the energy E=mDM/2. Such a monochromatic photon line can potentially be observed. In fact, in the simplest models sterile neutrino dark matter heavier than ~50 keV would produce a too large photon flux and is excluded. Thus the favored mass range for dark matter is between 1 and 50 keV. Then the photon line is predicted to fall into the X-ray domain that can be studied using X-ray satellites like XMM-Newton, Chandra, or Suzaku.

Until last week these searches were only providing lower limits on the lifetime of sterile neutrino dark matter. This paper claims they may have hit the jackpot. The paper use the XMM-Newton data to analyze the stacked X-ray spectra of many galaxy clusters where dark matter is lurking. After subtracting the background they see is this:

Although the natural reaction here is a resounding "are you kidding me", the claim is that the excess near 3.56 keV (red data points) over the background model is very significant, at 4-5 astrophysical sigma. It is difficult to assign this excess to any known emission lines from usual atomic transitions. If the excess is interpreted as a signal of new physics, one compelling (though not unique) explanation is in terms of sterile neutrino dark matter. In that case, the measured energy and intensity of the line correspond to the the neutrino mass 7.1 keV and the mixing angle of order 5*10^-5, see the red star in the plot. This is allowed by other constraints and, by twiddling with the lepton asymmetry in the neutrino sector, consistent with the observed dark matter relic density.

Clearly, a lot could possibly go wrong with this kind of analysis. For one thing, the suspected dark matter line doesn't stand alone in the spectrum. The background mentioned above consists not only of continuous X-ray emission but also of monochromatic lines from known atomic transitions. Indeed, the 2-10 keV range where the search was performed is pooped with emission lines: the authors fit 28 separate lines to the observed spectrum before finding the unexpected residue at 3.56 keV. The results depend on whether these other emission lines are modeled properly. Moreover, the known Ar XVII dielectronic recombination line happens to be nearby at 3.62 keV. The significance of the signal decreases when the flux from that line is allowed to be larger than predicted by models. So this analysis needs to be confirmed by other groups and by more data before we can safely get excited.

Decay diagrams borrowed from this review. For more up-to-date limits on sterile neutrino DM see this paper, or this plot. Update: another independent analysis of XMM-Newton data observes the anomalous 3.5 keV line in the Andromeda and the Perseus cluster.

Saturday, 15 February 2014

My blogging juices have been drying up for some time now, and at this point Résonaances is close to withering. This could be expected. The glorious year 2012 with all the excitement of the Higgs boson discovery was inevitably followed by post-coital depression, only amplified by the shutdown of the LHC for repairs.

One problem with blogging these days is that, in the short run, things are expected to get worse rather than better. The year 2013 was depressing but at least we could not complain of the lack of action. The LHC was flooding us with new results based on the data collected in the first run. On the Higgs front, the 125 GeV particle discovered the year before was established, beyond reasonable doubt, as a Higgs boson related to electroweak symmetry breaking. The CMB results from the Planck experiment were a sweeping victory for the Lambda-CDM description of the universe at large scales. The LUX experiment provided the best limits so far on the WIMP-nucleon cross section and slashed the hope that we may be on the verge of detecting dark matter. Plus a cherry on the top: ACME limits on electron's electric dipole moment increased the strain on any extension of the Standard Model with new particles at the TeV scale. Yes, a lot to remember, not much to cherish...

And what about 2014? Are there any results to be released this year that could be at least marginally exciting for particle physicists? I don't see much, and the opinion polls that I have conducted are not optimistic either. Basically, we just expect more of the same: the LHC, Planck, ICECUBE, AMS-02, Fermi... Of course, there is always a non-zero probability that some new results from these experiments will turn out to be a smoking gun for new physics, but the later in the game the dimmer the chances are. The only qualitatively new piece of data among those will be the Planck polarization data, but even that is unlikely to be a game-changer. One may also keep an eye on lightweight contenders: small precision experiments that pursue indirect limits on new physics. Recently there's been new such limits on non-standard interactions between electrons and quarks from JLab's PVDIS Collaboration who study low-energy scattering of electrons on nuclei. A similar experiment in JLab called Q-weak promises new results and improved limits this year. If there's anything else like that in the queue I'll be glad if you let me know in the comments section.

So how to live? How to blog? How to make it till 2015 when the sky is supposed to get brighter? I have no idea but I'll try to go on for a little longer. Back soon.

About Résonaances

Résonaances is a particle physics blog from Paris. It's about the latest news and gossips in particle physics and astrophysics. The posts are often spiced with sarcasm, irony, and a sick sense of humor. The goal is to make you laugh; if it makes you think too, that's entirely on your own responsibility...